Howlongcanhumanbeingslive？Mostscientistswhostudyoldagethinkthatthehumanbodyis（1）___tolivenolongerthan120years．However，110yearsisprobablythelongestthatanyonecouldhopetolive--ifheorsheis（2）___healthy
How long can human beings live？Most scientists who study old age think that the human body is（1）___ to live no longer than 120years．However，110years is probably the longest that anyone could hope to live--if he or she is （2）___ healthy and lucky．Some scientists even say we can live as long as 130years!Yet，our cells simply cannot continue to reproduce （3）___．They wear out，and as a result，we get old and （4）___ die．
Even though we can’t live forever，we are living a （5）___ life than ever before．In 1900，the average American life span （寿命） was only 47years，but today it is 75years!
When does old age begin then？Sixty-five may be out-of-date as the （6）___line between middle age and old age．After all，many older people don’t begin to experience physical and mental （7）___until after age 75．
People are living longer because more people （8）___ childhood．Before modern medicine changed the laws of nature，many children died of common childhood （9）___．Now that the chances of dying （10）___ are much lower，the chances of living long are much higher due to better diets and health care．
On the whole，our population is getting older．The（11）___ in our population will have lasting effects on our social development and our way of life．Some people fear such changes will be for the worse，while some see（12）___，not disaster，many men and women in their“golden years”are healthy，still active，and young in （13）___ if not in age．
As the society grows old，we need the （14）___ of our older citizens．With long lives ahead of them，they need to （15）___ active and devoted．
1．A．designed B．selected C．improved D．discovered
2．A．completely B．generally C．apparently D．extremely
3．A．rapidly B．harmlessly C．endlessly D．separately
4．A．eventually B．hopelessly C．automatically D．desperately
5．A．busier B．longer